1. Field
Exemplary embodiments of the present disclosure relate to a fixing unit and an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunctional device having at least two of the foregoing capabilities, employing the fixing unit.
2. Description of the Background Art
Known replaceable fixing units include a flexible endless rotatable fixing member, a substantially-cylindrical metal member (hereinafter referred to as a metal pipe) provided in proximity to an inner circumferential surface of the fixing member, a nip formation member held on the metal pipe in direct contact with the fixing member or in indirect contact therewith via a slide sheet, a lubricant applied to the nip formation member or the metal pipe, a rotatable pressing member disposed opposing and parallel to the nip formation member via the fixing member, and a heater to heat the fixing member via the metal pipe. In the fixing unit, heat and pressure are applied to a toner image at a fixing nip formed by the fixing member, the metal pipe, the nip formation member, the lubricant, and the pressing member to fix the toner image on a recording medium.
In such a fixing unit, grease may be applied to an outer circumferential surface of the metal pipe and a sliding portion of the nip formation member to reduce friction during rotation. Insufficient application of grease to the sliding portion of the nip formation member and the outer circumferential surface of the metal pipe risks damaging the ungreased areas. In addition, such insufficient application of grease may require a higher driving torque to rotatably drive the pressing member, thus imposing more load on a drive source.
Further, since the nip formation member is wrapped by a slide sheet made from, for example, a PTFE fabric and grease is retained between fibers of the fabric, the sliding portion of the nip formation member is well protected by grease. By contrast, for the metal pipe, it is necessary to spread grease over the entire metal pipe both in the axial direction and the circumferential direction thereof. If grease is not sufficiently spread over the entire metal pipe, a portion of the metal pipe over which grease is not spread may experience undue wear.
The above-described insufficiency of grease on the sliding portion and the metal pipe also refers not only to an absolute lack but also to a relative or localized lack of sufficient lubrication, that is, to a state in which, for example, when grease is additionally applied to the fixing unit before use or during maintenance, some portions are not covered with grease although grease must be applied over the entire metal pipe both in the axial and circumferential directions thereof. The reason why such a state occurs is that, because it is difficult and cumbersome for a person to apply grease uniformly to the entire metal pipe both in the axial and circumferential directions thereof, grease is applied to several positions (at least one position) in a dotted manner so that grease is naturally spread during operation of the fixing unit. In other words, there is a limitation in uniformly applying grease in such a way, resulting in the above-described insufficient spread state.
Therefore, in order to sufficiently spread grease, it is effective to rotate at least one of the portion to which grease is applied and the sliding portion that slides over the portion to which grease is applied so as to spread the grease over the entire metal pipe both in the axial and circumferential directions thereof. However, as far as the inventors of the present application are aware, there does not exist any fixing unit including a unit capable of spreading applied grease and any image forming apparatus including such a unit.
An additional factor involves grease viscosity. To prevent grease having a high viscosity at ordinary temperature from increasing the required torque, the conventional art heats grease in advance and then starts rotation, thus reducing the torque. However, grease is heated in a non-rotational state, and then printing is performed while rotationally heating the grease. As a result, when printing starts, grease is not sufficiently spread, resulting in the above-described failure.
In addition, a certain rotational state may not be obtained (for example, a torque of a certain level or less may not be obtained) due to variation in components of a fixing unit. Several reasons are possible for the failure in which a certain level or less of torque is not obtained. For example, when the parallelity between the fixing member (such as a pipe or a belt) and a pressure roller is lost, more grease may gather at one side in the axial direction than at the other side, thus preventing torque from attaining a certain level or less. When a gap between the outer circumferential surface of the metal pipe and the inner circumferential surface of the fixing belt is small (for example, due to defective components), the fixing belt and the metal pipe strongly contact each other, thus preventing the torque from attaining a certain level or less. In such a state, since it is useless to continue heating and rotation, it is desirable to stop operation and replace the fixing unit. However, as far as the inventors of the present application are aware, there is no conventional art for causing users to do such replacement.